Platelets are cellular fragments released into blood that function in hemostasis and thrombus formation. Megakaryocytes bear platelets through essential intermediate structures called proplatelets, microtubule based filopodial extensions with teardrop shaped ends where platelets are thought to assemble. Although proplatelet-based platelet production is widely accepted, the mechanisms involved in forming these structures are poorly understood. The objective of this proposal is to attain a greater understanding of the cytoskeletal events, particularly those involving microtubules, that underly proplatelet-based platelet production. Specific aims include identifying the sites of microtubule assembly in proplatelet-producing megakaryocytes by expressing and tracking fluorescently tagged proteins in living megakaryocytes, determining the mechanism of microtubule-based proplatelet elongation, defining the role of molecular motors on platelet elongation and establishing the polarity and organization of the proplatelet microtubule array.